GEOTECHNICAL ENGINEERING - TOPIC 3

This is the strength with which soil materials are held together or the resistance of soil to deformation and rupture.

Soil consistency

Four stages of consistency

Liquid state

Plastic state

Semi-solid state

Solid state

Identify the consistency of the soil based on the strength (kPa)

0-24 kPa

24-48 kPa

48-96 kPa

96-192 kPa

192-383 kPa

>383 kPa

Very soft

Soft

Medium

Stiff

Very stiff

Hard

When a fine-grained soil is mixed thoroughly with a large quantity of water, the resulting suspension is in a liquid state, and offers practically no resistance to flow. The soil has ____.

No shear strength

The water content of the suspension is gradually reduced to keep the consistency of the sample uniform. At this point, the soil starts to develop resistance to flow. The soil has ___ and change from liquid state to plastic state.

The soil can be molded into different shapes without rupturing because of plasticity.

Small shear strength

After further reducing the water content of the soil, the soil also loses plasticity and now becomes ___ .

The soil becomes brittle and molding the soil will result into cracking of the surface.

Semi-solid

In the ____, the soil cannot be remolded at all as it would lead to the soil specimen getting broken.

Solid state

The ___ are water contents at a certain limiting or critical stage in the soil behavior.

Atterberg limits

This is also known as the upper plastic limit, it is the water content at which the soil changes from the liquid state to a plastic state. It is the minimum moisture content at which a soil flows upon application of every small shear force.

Liquid limit

This is also known as the lower plastic limit, the water content at which the soil changes from the plastic state to a semi-solid state.

It is performed by repeatedly rolling an ellipsoidal-sized soil mass by hand on a non-porous surface. Casagrande defined the plastic limit as the water content at which a thread of soil just crumbles when it is carefully rolled out to a diameter of 3 mm (⅛”)

Plastic Limit

This is the arbitrary limit of water content at which the soil tends to pass from the semi-solid to solid state. It is the water content at which a soil, regardless of further drying, remains constant at volume.

Shrinkage limit

Formula for shrinkage limit

It is the ratio of the volume change of soil as a percentage of the dry volume to the corresponding change in moisture content.

Shrinkage ratio

formula for shrinkage ratio and specific gravity

It is the range of water content within which the soil exhibits plastic properties (i.e. difference between liquid and plastic limit)

Plasticity Index (PI = LL - PL)

It is the range if water content within which the soil is in a semisolid state of consistency (i.e. difference between the plastic and shrinkage limit)

Shrinkage Index (SI = PL - SL)

It is defined as the ratio of the difference between the liquid limit and the natural water content to the plastic limit of a soil.

Consistency Index (relative consistency)

It is defined as the ratio of the difference between the natural water content and the plastic limit to the plasticity index.

Liquidity Index (water-plasticity ratio)

Characteristics of Soil

LI < 0 Brittle Solid

LI < 1 Plastic

LI > 1 Liquid